To develop improved vaccines to measles that can safely overcome the maternal antibody barrier and induce long-lasting immunity, it is highly desirable to characterize the human humoral response to measles infection at the molecular level. This characterization can then form the basis for understanding the differences between humoral immunity induced by vaccination and by infection and for probing the specificities of maternal antibodies. Classical technologies can yield occasional human monoclonal antibodies but not in sufficient numbers or with sufficient reproducibility to consider characterizing antibody responses. Combinatorial library technology, i.e., antigen selection from antibody libraries generated in/on phage, offers rapid access to large numbers of human monoclonal antibodies (Fab fragments) and has the potential to allow analysis of human responses. The aim of this project is to use library technology to generate panels of recombinant Fabs against measles antigens from donors who have acquired immunity through natural infection. The Fabs will be characterized in terms of antigen recognized, affinity, amino acid sequence and neutralizing ability in vitro. The epitopes recognized will be defined in detail by competition with mouse antibodies and with other recombinant Fabs and by selection of non-neutralizable virus variants by antibody. The recombinant Fabs will then be used in competition studies with serum antibodies to compare the specificities of antibodies elicited by vaccination and infection and to describe the specificities of maternal antibodies.